It is known that an isocarbostyril derivative, 4-(3-tert.-butylamino-2-hydroxy)propoxy-2-methylisocarbostyril or an acid addition salts thereof such as the hydrochloride is especially useful for the therapeutic treatment of heart diseases (see: Japanese Patent Publication (KOKOKU) No. 41673/1978, U.K. Pat. No. 1501149 and U.S. Pat. No. 4,129,565). However, such a pharmaceutical composition containing 4-(3-tert.-butylamino-2-hydroxy)propoxy-2-methylisocarbostyril or an acid addition salt thereof as the active ingredient, in association with a pharmaceutically acceptable solid carrier for the active ingredient can always exhibit such disadvantages that the effective content of the isocarbostyril compound as the active ingredient is decreased or reduced during storage of the pharmaceutical composition with lapse of time, and that the whole composition is discolored into brown in appearance.
We, the present inventors, have made extensive researches in an attempt to eliminate the above-mentioned disadvantages usually observed with the pharmaceutical compositions comprising as the active ingredient the 4-(3-tert.-butylamino-2-hydroxy)propoxy-2-methylisocarbostyril or a pharmaceutically acceptable acid addition salt thereof together with the solid carrier for the active ingredient, and as a result we have now found that the isocarbostyril compound used as the active ingredient can be stabilized by adding thereto and mixing with an amount of one or more of such stabilizing agents which are chosen from magnesium silicate, magnesium oxide, hydrotalcite [Mg.sub.6 Al.sub.2 (OH).sub.16 CO.sub.3 4H.sub.2 O] and sodium hydrogen carbonate. We have thus accomplished this invention.